Contemporary sailing vessels employ semi-rigid, structural mast spars to support aloft the sail plan. Due to the mast cross-sectional dimensions necessary to support the compressional forces exerted on the mast by the rigging which holds the mast erect, a large dimensional reduction is presented to the air as it flows over the mast section and onto the thin membrane airfoil (sail). This large change in dimension from the mast to the sail surface causes the air stream to separate into turbulent eddies on the downstream side of the mast. The turbulence is sufficiently large as to cause a "separation bubble" to form along the leading edge, or luff, of the sail. The separation bubble increases the air drag on the sail and diminishes the surface area of the sail that experiences attached airflow (both laminar and turbulent) from which the greatest aerodynamic lift is obtained. Thus, the necessary presence of the mast, from a structural perspective, produces a negative influence on the lift-to-drag characteristics of the sail attached thereto.
Prior art approaches to solving this problem include designs which physically remove the mast from the vicinity of the sail, but those approaches suffer from extra structural weight and windage. Another approach which uses a "wrap-around-the-mast" soft (thin membrane) sail works only with masts not requiring spreaders and shrouds in the rigging; thus, it is unsuitable for medium-to-large high-performance vessels. Finally, some high-performance vessels have exploited rigid wings, also known as "hard sails." Hard sails work well, but are very expensive to fabricate and maintain, and they pose substantial problems for the vessel when not underway. Although the prior art does address the same problem, the solutions of the prior art involve complex and expensive designs appropriate to a small fraction of recreational sailing vessels. Most notably, as described below, inflatable sails and bags are employed, which require considerable alteration of the sail and/or mast.
Exemplary of the prior art, U.S. Pat. No. 3,132,620 to Court discloses a sailboat having alternative sail designs. In a first embodiment of a sail with two inflatable luff pockets, inflatable spaces are provided between the mainsail and two fairing sections each attached to opposite sides of the mainsail. The mainsail and each fairing section are attached to a rotatable mast bolt rope which engages bolt rope grooves in the mast. In a second embodiment, a fixed mast is provided with a double luff sail, each luff running around the mast to a slot near the front of the mast.
U.S. Pat. No. 4,753,186 to Paras is directed to a sailboat sail having sail panels which are wrapped around the mast. An inflatable bag having a sleeve is positioned within the panels with the sleeve over the mast.
U.S. Pat. No. 3,391,668 to Birchill is directed to a sail having a sealed chamber of lightweight flexible material extending along the leading edge portion, with an air inlet at the head of the sail. The chamber is inflatable by ram air entering the inlet. The chamber may be fitted around the mast or alternatively attached by a bolt rope to a bolt rope groove in the mast.
U.S. Pat. No. 4,947,776 to Peterson discloses an aerodynamically shaped fairing which fits around a sailboat headsail forestay. The fairing has a bolt rope groove for engaging the sail bolt rope at the leading edge of the head sail. An internal cavity is provided for pivotally carrying a toggle member which engages the forestay. A pair of inflatable tubes are disposed on opposite sides of the toggle member and are inflated selectively so as to flip the toggle member from side to side, thereby providing optimum leading edge camber of the sail on both port and starboard tacks.
U.S. Pat. No. 4,741,282 to Duke discloses an inflatable sail having either two outer sailcloth sides or panels or two outer panels and an inner panel, with inlet vents and outlet vents by which the airspace between the sailcloth sides is inflated by the wind.
U.S. Pat. No. 3,866,558 to Bergstrom et al. discloses a specific non-circular mast cross-section, having projections on the surface thereof a wedge-shaped enlarged slot on the rear thereof for receiving the luff edge of the sail. This patent shows and describes problems in airflow and flow separation.
The prior art also shows various approaches to airfoil improvement in sails, to wit:
U.S. Pat. No. 4,593,638 to Cochran et al. is directed to semi-rigid flexible profile members which fit around the mast. A profile sleeve formed in the sail is fitted over the profile members to form an aerodynamic wing-profile shape.
U.S. Pat. No. 4,479,451 to Lucht teaches a sail having a tubular air envelope portion at the front of the sail for running on a mast stay. A plurality of telescoping contour sleeves are disposed in the tubular portion to sheath the mast or stay when the envelope portion is run on the mast or stay.
U.S. Pat. No. 4,879,961 to Aguilera is directed to an inflatable airfoil device for extending over a substantial portion of an existing sail.
U.S. Pat. No. 4,803,939 to Jones discloses a slot-forming foil for mounting in front of the mast of a sailboat.
U.S. Pat. No. 4,785,757 to Ostholm is directed to an apparatus for stretching sails, which includes inflatable fluid-tight containers horizontally mounted at various elevations on the sail.
U.S. Pat. No. 4,646,671 to Innes et al. is directed to an airfoil for improving airflow past the leading edge of a sail, such as a headsail, by presenting a thick or relatively wide leading edge of the sail. For use in a headsail, the apparatus would not be mounted in the vicinity of the mast.
U.S. Pat. No. 4,388,888 to Gushurst, Jr. is directed to an adjustable airfoil and shows an aerodynamically shaped vane mounted on a mast which cooperates with luff slot flaps to make a smooth transition to flexible semi-rigid sail panels.
It should be noted that none of the inflatable prior art devices uses a relatively small inflatable tubular structure to support and maintain separation of luff panels in the vicinity of the aft portion of the mast. Rather, extensive inflatable bags, resulting in considerable sealing problems are used. Similiarly, inserts which do not require inflation are not taught by the prior art devices.